Hi, Ray Shore here. This page is dedicated to formulas related to telescope optics. I have listed the main formulas below for these calculations; each with a handy calculator. To the right, I have listed the specifications for my Schmidt-Cassegrain optical tube. The examples below will reference these specs.

Specifications for My C8 Optical Tube:

Aperture: 8" (203.2 mm)

Focal Ratio: f/10

Focal Length: 2032 mm

Highest Useful Magnification: 450X

Resolution: .57 arc/sec

1. Focal Ratio

Focal Length/Aperture

The smaller the f-ratio, the faster the telescope. Fast
telescopes (e.g. f/4 or f/6) provide wider fields of view (FOV) making them most
suitable for deep space observation.The larger the f-ratio, the slower the
telescope. The FOV becomes smaller as the f-ratio gets larger. Telescopes with
large f-ratio's (e.g. f/8 or f/10) are most suitable for high power planetary
work.

Example: my telescope has a large
f-ratio (f/10). To calculate, I take the focal length of 2032mm and divide by
it's aperture of 203.2mm. The result is f/10. My scope
is well suited for high planetary observation and imaging.

Focal Ratio Calculator

Focal Length:

aperture:

f-Ratio Result:

Important note: the focal length and aperture should be in the same units for this calculation. In my example above, I converted my 8" aperture to 203.2mm because the focal length was in mm.

Pertains to the resolving
power of an optical system (generally referred to when splitting double stars).
The higher the resolving power, the smaller the minimum visible detail or
minimum angle of resolution. The lower the resolving power, the bigger the
minimum visible detail or minimum angle of resolution.

Example: taking 4.56 arc
seconds and dividing by my scopes objective diameter of 8" gives me 0.57 arc/sec of
resolving power. This means that I could theoretically split a double star with
0.57 arc/seconds of separation.